Optical waveguides are structures that constrain or guide the propagation of light along a path defined by the physical construction of the guide. The dimensions of the guide in the directions in which the light is confined are on the order of the wavelength of the light. Such optical waveguides comprise a region of high refractive index in which most of the optical field of the light is located surrounded by regions of lower refractive index. Typically, an optical waveguide comprises a three-layer or sandwich structure comprising a substrate, a middle layer, often called a film, and a top layer or cover. The top layer or cover is very frequently air. The index of refraction is largest in the middle layer or film.
The middle layer or film has been made in the past from many materials using a variety of different techniques. For example, the film has been made from sputtered glass, sputtered oxides of tantalum or zinc, epitaxial gallium arsenide, ion-bombarded gallium arsenide, epitaxial garnets, sputtered and epitaxial lithium niobate, nitrobenzene liquid, and a number of other organic and polymeric materials.
Recently, there has been increased interest in the use of organic materials for optical devices. In Auston et al., "Research on Nonlinear Optical Materials: An Assessment", 26 Applied Optics, pp. 211-234 (1987), which is incorporated herein by reference, a review is presented on recent research into optical materials, including organic and polymeric materials. Conjugated polymers in particular, such as the polyacetylenes and polydiacetylenes, are known to have high third-order optical nonlinearities. Not only do these materials have extremely high third order hyperpolarizabilities, but they also have ultrafast (on the order of femtoseconds) response times. Thus, they are desirable materials for optical devices.
As described in Misin et al., "The Solid-Phase Polymerization of Monomers with Conjugated Acetylenic Groups", 56 Russian Chemical Reviews (1985), which is incorporated herein by reference, polydiacetylenes are prepared from diacetylene monomers by a solid state topotactic polymerization process. This process involves a direct transformation of crystalline diacetylene monomers having the general formula R--(C.dbd.C).sub.z --R' to crystalline polymer chains. The polydiacetylene crystals so synthesized are generally insoluble in common solvents, and only by elaborate crystallization techniques have polydiacetylenes suitable for optical devices been obtained.
Recently, several soluble polydiacetylenes have been synthesized and described in the literature. See, e.g., Muller et al., 185 Makromol. Chem. (1984), at p. 1727, and DE-OS No. 3347618 which are incorporated herein by reference. In the German patent document DE-OS No. 3346716, photolithographic methods are disclosed for making integrated electronic ciruits using polydiacetylenes as stable photoresists. According to the disclosure of this document, a soluble polydiacetylene is dissolved in a solvent, spin coated onto a silicon substrate, exposed to ultraviolet light through a mask, and developed in a given pattern to provide a precise resist image made from the polydiacetylene material.
Because of their excellent optical properties, it would be desirable to produce optical waveguides made from polydiacetylene materials. It would furthermore be desirable to produce optical waveguides made from polydiacetylene materials by a positive photolithographic process similar to the one described in the aforementioned DE-OS No. 3346716. However, the photolithographic process described in this patent document is not easily adapted to the manufacture of optical waveguides made from polydiacetylenes. This is because typical optical waveguides are about 0.8-2 microns, and because polydiacetylenes absorb ultraviolet light very strongly, the ultraviolet light which is used in the photolithographic process will not penetrate the polydiacetylene layer more than about 0.4 microns. It is therefore not possible to form a pattern in a polydiacetylene layer by such photolithographic techniques when the polydiacetylene layer is thicker than about 0.4 microns.
Accordingly, it is an object of the present invention to provide a novel method for producing a fine pattern in a polydiacetylene layer which is on a substrate.
It is another object of the present invention to provide a method for manufacturing optical waveguides of &gt;0.4 microns in thickness from polydiacetylene materials.
It is yet another object of the present invention to provide a method for manufacturing optical waveguides from polydiacetylene materials which method utilizes a positive photolithographic process in one of its preferred embodiments.